Flowable tissue products

ABSTRACT

The present disclosure provides tissue fillers. The tissue fillers can include a plurality of tissue particles formed from acellular tissue matrix fragments. The tissue fillers can be used to fill tissue sites, such as voids formed after tissue resection.

This application is a continuation of U.S. application Ser. No.15/377,481, filed Dec. 13, 2016, which is a continuation of U.S.application Ser. No. 13/717,808, granted as U.S. Pat. No. 9,549,805filed Dec. 18, 2012, which claims priority under 35 U.S.C. § 119 to U.S.Provisional Application No. 61/577,729, which was filed on Dec. 20,2011.

The present disclosure relates to tissue products, and moreparticularly, particulate tissue products for use as tissue fillers.

Various tissue products have been produced to replace, augment, or treattissue defects. For example, to replace or augment soft tissue defects,particulate acellular dermal matrices that form a paste or putty-likematerial can be used. Such products include, for example, CYMETRA®,which is a dermal acellular tissue matrix made by LIFECELL® Corporation(Branchburg, N.J.).

Although suitable for certain applications, further improvements in theability of tissue products for soft or hard tissue treatment aredesirable. The present disclosure describes improved tissue productsproduced from particulate tissue matrices.

SUMMARY

According to certain embodiments, a tissue product is provided. Theproduct can include a plurality of dry tissue matrix particlescomprising a longest dimension between about 1 mm and 5 mm. The tissuematrix particles can each comprise a plurality of tissue matrixfragments having a length between about 5 μm and 300 μm, wherein thetissue matrix fragments are formed into the tissue matrix particles.

According to certain embodiments, a method for producing a tissuetreatment composition is provided. The method can include selecting atissue matrix and treating the tissue matrix to produce fragments havinga length between about 5 μm and 300 μm. The method can further includeforming the fragments into a plurality of particles having a longestdimension between about 1 mm and about 5 mm; and treating the particlesto join the fragments forming each particle to one another. In someembodiments, the present disclosure includes tissue products producedaccording to the disclosed methods.

According to certain embodiments, a method of treating a tissue site isprovided. The method can comprise selecting a tissue site and selectinga tissue product, comprising a plurality of dry tissue particles,wherein the tissue matrix particles each comprise a plurality of tissuematrix fragments having a length between about 5 μm and 300 μm, andwherein the tissue matrix fragments are joined to one another to formthe tissue matrix particles. The method can further comprise placing theplurality of tissue particles in or on the tissue site.

According to certain embodiments, a tissue product is provided. Thetissue product can include plurality of dry tissue matrix particles thatform a flowable mass that can be poured into a tissue site and will flowto fill and conform to a tissue site. The particles are substantiallyspherical and have a radius between about 1 mm and 5 mm. The tissuematrix particles each comprise a plurality of tissue matrix fragmentshaving a length between about 5 μm and 300 μm, and the fragments arejoined to one another to form the tissue matrix particles.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process for producing a tissue product according tovarious embodiments.

DESCRIPTION OF CERTAIN EXEMPLARY EMBODIMENTS

Reference will now be made in detail to certain exemplary embodimentsaccording to the present disclosure, certain examples of which areillustrated in the accompanying drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

In this application, the use of the singular includes the plural unlessspecifically stated otherwise. In this application, the use of “or”means “and/or” unless stated otherwise. Furthermore, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Any range described herein will be understood toinclude the endpoints and all values between the endpoints.

The section headings used herein are for organizational purposes onlyand are not to be construed as limiting the subject matter described.All documents, or portions of documents, cited in this application,including but not limited to patents, patent applications, articles,books, and treatises, are hereby expressly incorporated by reference intheir entirety for any purpose.

As used herein “tissue product” will refer to any human or animal tissuethat contains extracellular matrix proteins. “Tissue products” caninclude intact tissue matrices, acellular or partially decellularizedtissue matrices, decellularized tissue matrices that have beenrepopulated with exogenous cells, and/or cellular tissues that have beenprocessed to change the orientation of at least some of the collagenfibers within the tissue's extracellular matrix.

Various tissue products are available for treatment of hard and softtissues. Such tissue products can include processed tissues, which havebeen treated to remove some or all of the cellular components and/orother materials (e.g., antigens and lipids). Such tissue products can beused for treatment, repair, regeneration, and/or augmentation of avariety of different tissues. For example, acellular tissue matrices canbe used to replace soft tissue lost or damaged due to, for example,surgery, trauma, disease, and/or atrophy.

Current tissue matrices or other tissue scaffold or replacementsmaterials (e.g., processed collagen or synthetic materials) areavailable in a variety of different forms. For example, STRATTICE™ andALLODERM® (LIFECELL® Corporation, Branchburg, N.J.) are two acellulardermal tissue matrix products that are sold as sheets. In addition,CYMETRA® (also from LIFECELL®) is a dry, particulate acellular dermalmatrix, which is produced by cryofracturing acellular dermis. Each ofthese materials can be used to treat various anatomic sites. STRATTICE™and ALLODERM® can be used for soft tissue augmentation, e.g., to treatabdominal wall defects; and CYMETRA® can be injected for soft tissueaugmentation.

Although some currently available tissue matrices are suitable fortreatment of certain anatomic sites, such materials may not be wellsuited for some applications. For example, when treating tissue defectsof varying size and geometry, e.g., after surgical excision of diseasedtissue, sheets may not be well suited to allow complete filling of atissue site. In addition, particulate materials may be packed or placedinto a tissue site (e.g., in the form of a paste or putty), but suchmaterials may not flow adequately to fill small defects, and may notmaintain sufficient porosity or space for rapid cellular infiltrationand formation of vascular structures. Accordingly, the presentdisclosure provides tissue products that can be used to fill tissuedefects having variable and/or irregular geometries. In addition, thetissue products of the present disclosure can provide suitableconfigurations to allow cellular ingrowth and vascular formation.

In various embodiments, a tissue product is provided. The tissue productcan include a plurality of dry tissue matrix particles. The particlescan be formed from tissue fragments that are joined to one another toproduce the desired particle size and shape. In various embodiments, theparticles comprise a longest dimension between about 1 mm and 5 mm andthe tissue matrix fragments that form the particles comprise a lengthbetween about 5 μm and 300 μm.

In various embodiments, a method for producing a tissue treatmentcomposition is provided. The method can include selecting a tissuematrix and treating the tissue matrix to produce fragments having alength between about 5 μm and 300 μm. The method can further compriseforming the fragments into a plurality of particles having a longestdimension between about 1 mm and about 5 mm.

In various embodiments, methods for treating a tissue site are provided.The methods can comprise selecting a tissue site and selecting a tissueproduct comprising a plurality of dry tissue particles, wherein thetissue matrix particles each comprise a plurality of tissue matrixfragments having a length between about 5 μm and 300 μm, and wherein thetissue matrix fragments are joined to one another to form the tissuematrix particles; and placing the plurality of tissue particles in or onthe tissue site.

In various embodiments a tissue product is provided. The tissue productcan comprise a plurality of dry tissue matrix particles that form aflowable mass that can be poured into a tissue site and will flow tofill and conform to the tissue site. The particles are substantiallyspherical and have a radius between about 1 mm and 5 mm. The tissuematrix particles each comprise a plurality of tissue matrix fragmentshaving a length between about 5 μm and 300 μm, wherein the tissue matrixfragments are joined to one another to form the tissue matrix particles.

In certain embodiments, the tissue products produced as described hereinprovide improved properties when implanted or during storage. Forexample, the products described herein may be less susceptible to damagecaused during freezing than other acellular tissue matrices. Inaddition, the matrices may have an improved ability to allow cellularingrowth and vascularization.

FIG. 1 illustrates a process for producing a tissue product according tovarious embodiments. As shown at step 101, the process begins withselecting a tissue matrix 100. Suitable tissue matrices are discussedfurther below, but the tissue matrices can include any substantiallyacellular tissue matrix produced from human or animal tissue, whichretains the ability to support cellular ingrowth and tissue regenerationwithout excessive inflammation. Certain exemplary tissue matrices thatmay be used include STRATTICE™ and ALLODERM® (LIFECELL® Corporation,Branchburg, N.J.), which are porcine and human acellular dermalmatrices, respectively. However, other suitable tissue matrices can beused, including, for example, small-intestine submucosa. In addition,the tissue matrices can include intact tissues (not decellularized)and/or tissues that have been partially decellularized and/or populatedwith exogenous cells.

Next, as shown at step 111, the matrix 100 is processed to producefragments 110. The tissue fragments 110 can be formed using a range ofsizes and different morphologies. For example, in some embodiments, thetissue fragments 110 are in the form of small strands or threads oftissue matrix that has been treated to produce the desired sizedistribution and/or shape. In various embodiments, the strands orthreads have a length between about 5 μm and 300 μm, between about 50 μmand 200 μm, between about 50 μm and 300 μm, or any values in between. Incertain embodiments, the strands are approximately 40 microns×140microns to 100 microns by 350 microns.

The tissue fragments 110 can be produced using a variety of processes.For example, any suitable cutting, grinding, milling, blending,shearing, or other mechanical process can be used, which produces thedesired size and shape and does not cause unsuitable damage or change tothe tissue matrix. In certain embodiments, the tissue fragments 110 areprocessed using a mill such as a SYMPAK® food mill or a QUADRO AttritionMill (Quadro, Canada). In some embodiments, the tissue matrix 100 is cutinto small pieces (e.g., 4 cm×4 cm) and then milled. In addition, thematrix may be blended briefly in a solution (e.g., PBS) prior tomilling.

In some cases, the tissue matrices 100 can be processed to produce thefragments 110 when wet or submerged in a liquid. For example, the tissuematrices 100 can be milled or otherwise processed when submerged in abuffer such as PBS or any other suitable buffer. Further, afterprocessing, the buffer can be at least partially removed by centrifugingor filtering to remove some or all of the liquid component. For example,a suitable centrifugation protocol can include centrifuging at 4,500rpms for about 60 min.

After processing to produce tissue fragments 110, groups of thefragments 120 are formed to produce particles 120 having a desiredshape, as shown at Step 121. The specific shapes and sizes of theparticles 120 can vary based on the intended implantation site, tocontrol space between particles to provide channels for cellular andvascular ingrowth, or to control the ability of the particles to flowinto a desired treatment site. The tissue particles 120 can be shapedusing a variety of molding or shaping processes. For example, thefragments 110 may be placed into a mold and/or compressed, rolled into adesired shape, or otherwise manually manipulated to produce the desiredshape.

In some embodiments, the particles can be formed by immersion in a coldliquid. For example, fragments containing a buffer such as PBS can beextruded from a syringe and slowly dropped into liquid nitrogen. Thematerial, when dropped into liquid nitrogen will form small particles,and the relative dimensions of the particles can be controlled bycontrolling the speed of extrusion and water content of the materials.

In some cases, after extrusion, the materials can be further processedto produce a desired shape and/or structure. For example, in some cases,the frozen materials are placed into a mixing device, such as a panner.A panner is a cooking attachment to a mixer, which acts like a rocktumbler or cement mixer; it rotates at a given speed and tumbleswhatever objects are inside to achieve a coating of whatever powder orliquid is added. However, other similar mixing devices can be used.After placement in the mixing device, additional dry strands produced asdiscussed above may be added to the mixing device (e.g., atapproximately a 1:1 ratio of particles and dry strands). The materials,with or without the additional dry strands can be processed in thepanner or similar mixing device to produce a more spherical shape, andor change the size of the particles]. Optionally, the particles may beat least partially dried while in the panner. For example, the frozenparticles in the mixing device can be exposed to low levels of hot air(e.g., approximately 48° C. as a velocity that does not blow theparticles out of the processing device). As the particles in the mixingdevice are slowly heated and dried, additional tissue fragments in theform of dry powder may be added to keep the particles coated. Adding thedry powder, in this way, can assist in pulling residual moisture to thesurface of the particles to dry the interior. Optionally, the particlesmay be further dried within the mixing device to remove most moisture.

A variety of shapes can be used for the tissue particles 120. Forexample, the tissue particles 120 can be formed into substantiallyspherical shapes, oblong shapes (e.g., ovoid), cubes, rectangles,noodles, pyramids, or any other desired shape. In some embodiments, theshape is selected to control flowability when implanted. For example,spherical shapes may be selected to allow a high degree of flowability.Alternatively, more oblong shapes may be selected to allow filling of aspace while preventing migration out of a desired location. In addition,the specific shape may be selected to control the space betweenparticles. For example, a spherical shape and size may be selected toproduce a certain amount of porosity to allow cellular ingrowth and/orformation of vascular or extracellular structures.

In addition, the size of the particles can be varied based on a desiredapplication. For example, the particles may have a longest dimensionbetween about 1 mm and about 5 mm. Therefore, if the particles arespherical, the particles will have a diameter between about 1 mm and 5mm, and if the particles are ovoid, the particles will have a long axiswith a length between about 1 mm and 5 mm.

In various embodiments, the particles are processed such that thefragments making up the particles are joined to one another to formstable structures, as shown at Step 131. In certain embodiments, thefragments are joined without the use of substantial amounts of binder oradhesives. In addition in some embodiments, the fragments are driedusing a process that is believed to join the fragments withoutsignificant cross-linking. For example, in some cases, the fragments mayhave frayed ends that interlock with one another. Further, in someembodiments, the fragments may bind to one another by non-covalentbinding. As discussed elsewhere, the particles may be dried using aprocess such as convective drying, and such processes can produceparticles having fragments that are joined to one another.

In some embodiments, the fragments are joined to one another bycross-linking. Cross-linking can be accomplished using a number ofprocesses such as dehydrothermal cross-linking, exposure to UV light,and/or chemical cross-linking. In some embodiments, a dehydrothermalcross-linking process is used to allow cross-linking whilesimultaneously drying the particles. In addition, using any of thecross-linking processes, the particles may be further dried (e.g., byfreeze-drying or air drying) to remove additional moisture.

In various embodiments, the tissue products can be selected to havecertain properties that facilitate implantation and tissue fillingand/or regeneration. For example, in certain embodiments, the tissueparticles are dry before implantation. The dry particles can form aflowable mass that will fill a void or pocket in a tissue site. Thetissue particles can be dried by freeze-drying and/or concurrently witha dehydrothermal cross-linking process. In addition, in the particlescan be selected such that they swell when contacted with an aqueousenvironment, as may be present in a tissue site. As such, the particlescan expand when implanted to fill a selected tissue site.

In some embodiments, the particles are dried by convective heating. Forexample, frozen particles may be placed in a convection dryer (e.g.,HARVEST Brand Kitchen Convection Dryer). Drying may be performed atapproximately 45° C. However, lower or higher temperatures may be used,as long as temperatures that cause unacceptable denaturation or othertissue damage are not used. In addition, it should be noted, that evenwhen partially or mostly dried, as described above using a panner, theparticles may be further dried to remove excess moisture.

After drying, the particles are packaged and sterilized to form a finalproduct 140, as shown at Step 141. The product can be package in avariety of known medical containers and can be sterilized usingconventional processes as long as the processes do not damage theproduct (e.g., by excessive cross-linking) in an unacceptable manner. Insome embodiments, the product can be packaged in foil-to-foil pouchesand irradiated. In some embodiments, the product can be irradiated withe-beam radiation. Suitable e-beam doses can include 15-22 kGy or rangestherebetween.

The tissue products of the present disclosure can be used to treat avariety of different soft tissue or hard tissue sites. For example, theproducts can be used to replace, repair, regenerate or augment tissuelost or destroyed due to surgery, trauma, and/or any pathologic process.In some embodiments, the tissue products can be implanted in a softtissue site such as a lumpectomy site. In other embodiments, theproducts can be used to treat or augment bone, muscle, subcutaneoustissue, and/or adipose tissue.

In certain embodiments, internal negative pressure can be applied withinthe tissue product. In certain embodiments, negative pressure can serveto draw cells from surrounding tissue into the implanted acellulartissue product, increasing the rate at which native cells migrate intothe tissue product and enhancing the speed and/or overall effectivenessof tissue approximation.

In certain exemplary embodiments, internal negative pressure isdelivered to the acellular tissue matrix by a reduced pressure therapydevice. The reduced pressure therapy device can include a pump fluidlyconnected, e.g., through a fluid passage or tubing to the acellulartissue matrix, and which delivers reduced or negative pressure to theacellular tissue matrix. A variety of reduced pressure therapy devicescan be used. For example, suitable reduced pressure therapy devicesinclude V.A.C.® therapy devices produced by KCl (San Antonio, Tex.).

Acellular Tissue Matrices

The term “acellular tissue matrix,” as used herein, refers generally toany tissue matrix that is substantially free of cells and/or cellularcomponents. Skin, parts of skin (e.g., dermis), and other tissues suchas blood vessels, heart valves, fascia, cartilage, bone, and nerveconnective tissue may be used to create acellular matrices within thescope of the present disclosure. Acellular tissue matrices can be testedor evaluated to determine if they are substantially free of cell and/orcellular components in a number of ways. For example, processed tissuescan be inspected with light microscopy to determine if cells (live ordead) and/or cellular components remain. In addition, certain assays canbe used to identify the presence of cells or cellular components. Forexample, DNA or other nucleic acid assays can be used to quantifyremaining nuclear materials within the tissue matrices. Generally, theabsence of remaining DNA or other nucleic acids will be indicative ofcomplete decellularization (i.e., removal of cells and/or cellularcomponents). Finally, other assays that identify cell-specificcomponents (e.g., surface antigens) can be used to determine if thetissue matrices are acellular.

In general, the steps involved in the production of an acellular tissuematrix include harvesting the tissue from a donor (e.g., a human cadaveror animal source) and cell removal under conditions that preservebiological and structural function. In certain embodiments, the processincludes chemical treatment to stabilize the tissue and avoidbiochemical and structural degradation together with or before cellremoval. In various embodiments, the stabilizing solution arrests andprevents osmotic, hypoxic, autolytic, and proteolytic degradation,protects against microbial contamination, and reduces mechanical damagethat can occur with tissues that contain, for example, smooth musclecomponents (e.g., blood vessels). The stabilizing solution may containan appropriate buffer, one or more antioxidants, one or more oncoticagents, one or more antibiotics, one or more protease inhibitors, and/orone or more smooth muscle relaxants.

The tissue is then placed in a decellularization solution to removeviable cells (e.g., epithelial cells, endothelial cells, smooth musclecells, and fibroblasts) from the structural matrix without damaging thebiological and structural integrity of the collagen matrix. Thedecellularization solution may contain an appropriate buffer, salt, anantibiotic, one or more detergents (e.g., TRITON X-100™, sodiumdeoxycholate, polyoxyethylene (20) sorbitan mono-oleate), one or moreagents to prevent cross-linking, one or more protease inhibitors, and/orone or more enzymes. In some embodiments, the decellularization solutioncomprises 1% TRITON X-100™ in RPMI media with Gentamicin and 25 mM EDTA(ethylenediaminetetraacetic acid). In some embodiments, the tissue isincubated in the decellularization solution overnight at 37° C. withgentle shaking at 90 rpm. In certain embodiments, additional detergentsmay be used to remove fat from the tissue sample. For example, in someembodiments, 2% sodium deoxycholate is added to the decellularizationsolution.

After the decellularization process, the tissue sample is washedthoroughly with saline. In some exemplary embodiments, e.g., whenxenogenic material is used, the decellularized tissue is then treatedovernight at room temperature with a deoxyribonuclease (DNase) solution.In some embodiments, the tissue sample is treated with a DNase solutionprepared in DNase buffer (20 mM HEPES(4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), 20 mM CaCl₂ and 20mM MgCl₂). Optionally, an antibiotic solution (e.g., Gentamicin) may beadded to the DNase solution. Any suitable buffer can be used as long asthe buffer provides suitable DNase activity.

While an acellular tissue matrix may be made from one or moreindividuals of the same species as the recipient of the acellular tissuematrix graft, this is not necessarily the case. Thus, for example, anacellular tissue matrix may be made from porcine tissue and implanted ina human patient. Species that can serve as recipients of acellulartissue matrix and donors of tissues or organs for the production of theacellular tissue matrix include, without limitation, mammals, such ashumans, nonhuman primates (e.g., monkeys, baboons, or chimpanzees),pigs, cows, horses, goats, sheep, dogs, cats, rabbits, guinea pigs,gerbils, hamsters, rats, or mice.

Elimination of the α-gal epitopes from the collagen-containing materialmay diminish the immune response against the collagen-containingmaterial. The α-gal epitope is expressed in non-primate mammals and inNew World monkeys (monkeys of South America) as well as onmacromolecules such as proteoglycans of the extracellular components. U.Galili et al., J. Biol. Chem. 263: 17755 (1988). This epitope is absentin Old World primates (monkeys of Asia and Africa and apes) and humans,however. Id. Anti-gal antibodies are produced in humans and primates asa result of an immune response to α-gal epitope carbohydrate structureson gastrointestinal bacteria. U. Galili et al., Infect. Immun. 56: 1730(1988); R. M. Hamadeh et al., J. Clin. Invest. 89: 1223 (1992).

Since non-primate mammals (e.g., pigs) produce α-gal epitopes,xenotransplantation of collagen-containing material from these mammalsinto primates often results in rejection because of primate anti-Galbinding to these epitopes on the collagen-containing material. Thebinding results in the destruction of the collagen-containing materialby complement fixation and by antibody dependent cell cytotoxicity. U.Galili et al., Immunology Today 14: 480 (1993); M. Sandrin et al., Proc.Natl. Acad. Sci. USA 90: 11391 (1993); H. Good et al., Transplant. Proc.24: 559 (1992); B. H. Collins et al., J. Immunol. 154: 5500 (1995).Furthermore, xenotransplantation results in major activation of theimmune system to produce increased amounts of high affinity anti-galantibodies. Accordingly, in some embodiments, when animals that produceα-gal epitopes are used as the tissue source, the substantialelimination of α-gal epitopes from cells and from extracellularcomponents of the collagen-containing material, and the prevention ofre-expression of cellular α-gal epitopes can diminish the immuneresponse against the collagen-containing material associated withanti-gal antibody binding to α-gal epitopes.

To remove α-gal epitopes, after washing the tissue thoroughly withsaline to remove the DNase solution, the tissue sample may be subjectedto one or more enzymatic treatments to remove certain immunogenicantigens, if present in the sample. In some embodiments, the tissuesample may be treated with an α-galactosidase enzyme to eliminate α-galepitopes if present in the tissue. In some embodiments, the tissuesample is treated with α-galactosidase at a concentration of 300 U/Lprepared in 100 mM phosphate buffer at pH 6.0. In other embodiments, theconcentration of α-galactosidase is increased to 400 U/L for adequateremoval of the α-gal epitopes from the harvested tissue. Any suitableenzyme concentration and buffer can be used as long as sufficientremoval of antigens is achieved.

Alternatively, rather than treating the tissue with enzymes, animalsthat have been genetically modified to lack one or more antigenicepitopes may be selected as the tissue source. For example, animals(e.g., pigs) that have been genetically engineered to lack the terminalα-galactose moiety can be selected as the tissue source. Fordescriptions of appropriate animals see co-pending U.S. application Ser.No. 10/896,594 and U.S. Pat. No. 6,166,288, the disclosures of which areincorporated herein by reference in their entirety. In addition, certainexemplary methods of processing tissues to produce acellular matriceswith or without reduced amounts of or lacking alpha-1,3-galactosemoieties, are described in Xu, Hui. et al., “A Porcine-Derived AcellularDermal Scaffold that Supports Soft Tissue Regeneration: Removal ofTerminal Galactose-α-(1,3)-Galactose and Retention of Matrix Structure,”Tissue Engineering, Vol. 15, 1-13 (2009), which is incorporated byreference in its entirety.

After the acellular tissue matrix is formed, histocompatible, viablecells may optionally be seeded in the acellular tissue matrix to producea graft that may be further remodeled by the host. In some embodiments,histocompatible viable cells may be added to the matrices by standard invitro cell co-culturing techniques prior to transplantation, or by invivo repopulation following transplantation. In vivo repopulation can beby the recipient's own cells migrating into the acellular tissue matrixor by infusing or injecting cells obtained from the recipient orhistocompatible cells from another donor into the acellular tissuematrix in situ. Various cell types can be used, including embryonic stemcells, adult stem cells (e.g. mesenchymal stem cells), and/or neuronalcells. In various embodiments, the cells can be directly applied to theinner portion of the acellular tissue matrix just before or afterimplantation. In certain embodiments, the cells can be placed within theacellular tissue matrix to be implanted, and cultured prior toimplantation.

What is claimed is:
 1. A method for producing a tissue composition,comprising: selecting a tissue matrix; treating the tissue matrix toproduce fragments having a length between about 5 μm and 300 μm; andforming the fragments into a plurality of particles, each particlehaving a longest dimension between about 1 mm and about 5 mm.
 2. Themethod of claim 1, further comprising drying the particles.
 3. Themethod of claim 2, wherein drying the particles includes subjecting theparticles to a convective drying process.
 4. The method of claim 1,further including treating the particles with a dehydrothermal treatmentprocess.
 5. The method of claim 1, wherein forming the fragments into aplurality of particles includes compressing groups of the fragments toproduce each particle.
 6. The method of claim 1, wherein forming thefragments into a plurality of particles includes placing small groups ofthe fragments in a cold environment to freeze the groups.
 7. The methodof claim 6, wherein placing small groups of the fragments in a coldenvironment to freeze the groups includes extruding the small groupsinto a cryogenic liquid.
 8. The method of claim 1, wherein the pluralityof particles includes substantially spherical particles.
 9. The methodof claim 1, wherein treating the tissue matrix to produce fragmentsincludes milling the tissue matrix.
 10. The method of claim 1, whereinthe fragments are strands of tissue matrix.
 11. The method of claim 1,wherein the tissue matrix is dermal tissue matrix.
 12. The method ofclaim 1, wherein the tissue matrix is porcine tissue matrix.
 13. Themethod of claim 1, wherein the particles are flowable.
 14. The method ofclaim 1, wherein the tissue matrix is an acellular tissue matrix.
 15. Atissue product, comprising: a plurality of dry tissue matrix particlesthat form a flowable mass configured to fill and conform to a tissuesite, wherein the particles have a radius between about 1 mm and 5 mm,wherein the tissue matrix particles each comprise a plurality of tissuematrix fragments having a length between about 5 μm and 300 μm, andwherein the tissue matrix fragments are joined to one another to formthe tissue matrix particles.
 16. The tissue product of claim 15, whereinthe tissue matrix particles include dermal tissue matrix particles. 17.The tissue product of claim 15, wherein the tissue matrix particlesinclude porcine tissue matrix particles.
 18. The tissue product of claim15, wherein the particles swell when contacted with water.
 19. Thetissue product of claim 15, wherein the particles are substantiallyspherical.